It is known for several years that supramolecular polymers are polymers in which the monomers are at least in part bond to one another via H-bridges.
When the monomer units have a low molecular weight, they form at low temperature a rigid dimensionally stable polymer. At higher temperatures, however, since the H-bridges are much weaker, essentially only monomeric units are present and can be easily handled.
International Patent Application No. WO 97/46607 relates to a supramolecular polymer containing monomeric units that form H-bridges with one another, the H-bridge-forming monomeric units in pairs forming at least 4-H-bridges with one another. As H-bridge-forming monomeric units, substituted ureido-pyrimidones and ureido-pyrimidines were used. In examples XII and XIII of that International Patent Application is described the end-capping of polydimethyltrisiloxanes with 4-benzyloxy-6-(3-butenyl)-2-butylureidopyrimidine and 6-(3-butenyl)-2-butylureido-4-pyrimidone, respectively.
Supramolecular polymers have also been described in our co-pending PCT application PCT/EP01/14082.
In “Reversible Polymers Formed from Self-Complementary Monomers Using Quadruple Hydrogen Bonding”, by R. P. Sijbesma, H. B. Beijer, L. Brunsveld, B. J. B. Folmer, J. H. K. Ky Hirschberg, R. F. M. Lange, J. K. L. Lowe, E. W. Meijer, published in Science, Vol. 278, 28 Nov. 1997, is described in FIG. 2 the reaction of 6-tridecylisocytosine with hexanediisocyanate to give a bifunctional compound (2a) which forms reversible polymers. In FIG. 6 of that reference is also disclosed the functionalization of a trifunctional copolymer of propylene oxide and ethylene oxide with a diisocyanate, followed by a reaction with methylisocytosine to give a compound (7) which has the ability to form reversible polymer networks. These compounds (2a) and (7) are supposed to allow the formation of polymer networks that can be used in hot melts and coatings. However, as indicated in the reference, compound (2a) has a tendency to crystallise and compound (7) exhibits poor mechanical properties.
In “New Polymers Based on the Quadruple Hydrogen Bonding Motif”, by Brigitte J. B. Folmer, pages 91-108, PhD Thesis, Technische Universiteit Eindhoven, 2000, is described (see in particular page 96) the end-capping of hydroxy terminated polymers with a reactive synthon obtained by the reaction of methylisocytosine with 1,6-hexanediisocyanate. The hydroxy terminated polymers are a hydrogenated polybutadiene, a polyether, a polycarbonate and a polyester.
As a result of the increased demand for lighter thermoplastic materials, a low density TPU needs to be developed which, in turn, represents a big technical challenge to provide, at minimum, equal physical properties to conventional low density PU.
It is already known to produce soles and other parts of polyurethane by a polyaddition reaction of liquid reactants resulting in an elastic solid moulded body. Up till now, the reactants used were polyisocyanates and polyesters or polyethers containing OH-groups. Foaming was effected by adding a liquid of low boiling point or by means of CO2, thereby obtaining a foam at least partially comprising open cells.
Reducing the weight of the materials by foaming the TPU has not given satisfactory results up to now. Attempts to foam TPU using well-known blowing agents as azodicarbonamides (exothermic) or sodiumhydrocarbonate (endothermic) based-products were not successful for mouldings with reduced densities below 800 kg/m3.
With endothermic blowing agents, a good surface finish can be obtained but the lowest density achievable is about 800 kg/m3. Also, the processing is not very consistent and results in long demoulding times. Very little or no foaming is obtained at the mould surface due to a relatively low mould temperature, resulting in a compact, rather thick skin and a coarse cell core.
By using exothermic blowing agents, a lower density foam (down to 750 kg/m3) with very fine cell structure can be achieved but the surface finish is not acceptable for most applications and demould time is even longer.
From the above it is clear that there is a continuous demand for low density TPUs having improved skin quality which can be produced with reduced demould times.
It has now been surprisingly found that foaming supramolecular polymers with a PU backbone structure in the presence of blowing agents, allows one to meet the above objectives. Demould times are significantly reduced and the process can be carried out at lower temperatures, resulting in a better barrel stability. In addition, the use of blowing agents, even allows to further reduce the density while maintaining or improving the skin quality and demould time. Alternatively, the efficiency of the blowing agents, when combined with the supramolecular polymer is improved.
The present invention thus concerns a process for the preparation of foamed supramolecular polymers whereby the foaming of the supramolecular polymer is carried out in the presence of blowing agents.
The low density supramolecular polymers thus obtained (density not more than 800 kg/m3) have a fine cell structure, very good surface appearance, a relatively thin skin and show comparable physical properties to conventional PU which renders them suitable for a wide variety of applications.
The invention provides TPU products having outstanding low temperature dynamic flex properties and green strength at the time of demould, at density 800 kg/m3 and below.
The term “green strength”, as is known in the art, denotes the basic integrity and strength of the TPU at demould. The polymer skin of a moulded item, for example, a shoe sole and other moulded articles, should possess sufficient tensile strength and elongation and tear strength to survive a 90 to 180 degree bend without exhibiting surface cracks. The prior art processes often require 5 minutes minimum demould time to attain this characteristic.
In addition, the present invention therefore provides a significant improvement in minimum demould time. That is to say, a demould time of 2 to 3 minutes is achievable.
Adding blowing agents during the processing of TPUs is widely known, see e.g. WO-A 94/20568, which discusses the production of foamed TPUs, in particular expandable, particulate TPUs, EP-A 516024, which describes the production of foamed sheets from TPU by mixing with a blowing agent and heat-processing in an extruder, and DE-A 4015714, which concerns glass-fibre reinforced TPUs made by injection moulding TPU mixed with a blowing agent.
The use of microspheres in a polyurethane foam has been described in EP-A 29021 and U.S. Pat. No. 5,418,257.
Nevertheless, none of the prior art documents discloses the use of a combination of a supramolecular polymer and blowing agents to produce foamed low density TPU (density 800 kg/m3 and even below) nor do these documents suggest the benefits associated with the present invention including the skin quality and thereby maintaining physical properties at low density.